The present invention relates to flexible pipelines used for transporting fluids, such as, for example, live crude oil, along the seabed between sub-sea operating installations or between sub-sea or surface operating installations and coastal operating installations. Such flexible pipelines are of the type known as flow lines.
Flexible pipelines of the flow line type may, in certain cases, be protected when used on seabed by being covered with a certain depth of sediment or of a filling material, known as depth of cover, so as to be protected from any damage due to shipping or to the environment or for the purpose of not being subjected to the action of the current and/or the swell.
This protection may typically be achieved by digging a trench or furrow in the seabed, placing the flexible pipeline in this trench, then allowing the trench to fill naturally or, preferably, by filling the trench. This is then known as burying.
There are also other ways of protecting a flexible pipeline, for example covering the flexible pipeline, laid on the seabed, with rocks or any other filling material then known as rock dumping or gravel dumping, or with bags filled with gravel or cement as described in patent FR 2,602,300. This is then known as sand bagging.
In all cases, the problem of a variation in the length of the flexible pipeline thus protected, under the effect of the internal pressure or of expansions of thermal origin (as temperature variations may reach 100.degree. C. and more) arises, because the buried or covered flexible pipeline is not free to deform laterally or downwards, and its extension, localized to individual regions where the depth of cover is less or there is fluidized sediment, may then lead to the formation of one or more buckling loops of a significant height and to the emergence of all or part of the flexible pipeline, something which is of course unacceptable.
Reference will usefully be made to the publication by the French Petroleum Institute entitled "Flambage vertical des conduites ensouillees [vertical buckling in buried pipelines]", published in the magazine of the French Petroleum Institute, volume 37, No. 1, January/February 1982, which examines from the theoretical and the experimental point of view the risk of buckling in flexible pipelines and of the formation of at least one buckling loop, as a function of the pressure inside the flexible pipeline and of the stiffness of this pipeline.
Since 1980, the Applicant Company has manufactured and laid several hundreds of kilometers of buried or covered flexible pipeline. These pipelines all have a reinforcing layer for withstanding circumferential components of the internal pressure, called a pressure arch, and reinforcing layers for mainly withstanding tension, known as tension reinforcing layers, and tend to lengthen under the effect of an increase in internal pressure. The tension reinforcing layers are overspecified in order to give the pipeline a significant stiffness and benefit from the fact that the stiffer the pipeline, the higher the critical pressure, that is to say the internal pressure to which the flexible pipeline must be subjected in order to cause a buckling loop to appear.
Despite their special design, these known flexible pipelines have to be buried under a significant depth of cover, to avoid any risk of one or more buckling loops emerging.
By way of indication, for a pipeline inside diameter of between 2 and 18 inches, and a service pressure of between 100 and 400 bar, the depth of cover is usually between 1 m 50 and 2 m 50 and the pipelines are therefore considerably time-consuming and expensive to lay.
To avoid the appearance of buckling loops, efforts have been made to design flexible pipelines that are dimensionally stable in terms of length, specially intended for burying.
Thus, publication U.S. Pat. No. 5,024,252 proposes a flexible pipeline especially intended for burying, of the rough bore type, comprising, from the center outwards:
an unsealed crush-resistant reinforcing layer, also known as an internal carcass, typically consisting of a helical winding of a clipped profile, for example a steel tape, PA1 at least one polymeric layer covering the internal carcass and constituting a sealing barrier, PA1 two pairs of crossed reinforcing plies of helically-wound steel wires, PA1 a protective external polymeric jacket. PA1 an internal carcass, PA1 a sealing barrier, PA1 a reinforcing layer which mainly withstands the circumferential component of the internal pressure, and consists of a helical winding of a steel wire at a helix angle greater than 85.degree. with respect to the longitudinal direction of the pipeline, and of stiffness K.sub.p, PA1 two crossed plies of steel wires wound in a helix at an angle at most equal to 55.degree., mainly providing resistance to tension, and of stiffness K.sub.t, PA1 an outer protective jacket.
The reinforcing plies of each pair are crossed, that is to say that the wires which form them are wound with opposing helix angles with respect to the longitudinal direction of the pipeline, so that the pipeline is stable in torsion, and the winding angles are chosen to give the pipeline dimensional stability in the direction of its length. The wires of one of the pairs of crossed plies are thus wound at a helix angle of between 40.degree. and 53.degree., and the wires of the other pair are wound at a helix angle of between 57.degree. and 70.degree., the said helix angles being measured with respect to the longitudinal direction of the pipeline.
There has also been proposed, in publication EP 147,288, a flow line specially intended to be buried, also of the rough bore type, comprising, from the center outwards:
The ratio K.sub.t /K.sub.p is high, typically greater than or equal to 3, in order to give the pipeline lengthwise stability.
The flexible pipelines proposed in the aforementioned two patents are relatively expensive. A flexible pipeline such as described in patent EP 147,288 may even tend to shorten under the effect of an increase in the internal pressure and to lengthen when it returns to a lower pressure, depending on the value of the ratio K.sub.t /K.sub.p. Thus, when such a flexible pipeline is laid in a trench running across a dip in the seabed, and describes a downwards loop to follow the concaveness of the bed, it tends, when pressurized, to shorten and to move out of the trench. Once it is out of its trench, the pipeline lengthens, in the event of a reduction in the internal pressure, and forms an emerging buckling loop.